-
Metamorphic Facies Map of Southeastern Alaska- Distribution,
Facies, and Ages of Regionally Metamorphosed Rocks
By CYNTHIA DUSEL-BACON, DAVID A. BREW, and SUSAN L. DOUGLASS
REGIONALLY METAMORPHOSED ROCKS O F ALASKA
U . S . G E O L O G I C A L S U R V E Y P R O F E S S I O N A L
P A P E R 1 4 9 ' 7 - D
Prepared i n cooperation with the Alaska Department of Natural
Resources, Division of Geological and Geophysical survey^
U N I T E D S T A T E S G O V E R N M E N T P R I N T I N G O F
F I C E , W A S H I N G T O N : 1 9 9 6
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U.S. DEPARTMENT OF THE INTERIOR
BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY
Gordon P. Eaton, Director
Any use of trade, product, or firm names in this publication is
for descriptive purposes only and does not imply endorsement
by the U.S. Government
Text edited by Jan Detterman Illustrations edited by Jan
Detterman and Dale Russell, prepared
by Mike Newman and Glen Schumacher
Library of Congress Catalog in Publication Data
Dusel-Bacon, Cynthia. Metamorphic facies map of southeastern
Alaska: Distribution,
facies, and ages of regionally metamorphosed rocks 1 by Cynthia
Dusel-Bacon, David A. Brew, and Susan L. Douglass.
p. cm. -- (Regionally metamorphosed rocks of Alaska) (U.S.
Geological Survey Professional Paper ; 1497-D)
"Prepared in cooperation with the Alaska Department of Natural
Resources, Division of Geological and Geophysical Surveys."
Includes bibliographical references (p. - 1. Supt. of Docs. no.
: 1 19. 16: 1497-D 1. Rocks, Metamorphic--Alaska. 2. Metamorphism
(geology)--Alaska.
I. Brew, David A. 11. Douglass, Susan L. 111. Alaska. Division
of Geological and Geophysical Surveys. IV. Title. V. Series. VI.
Series: U.S. Geological Survey professional paper ; 1497-D.
QE475.A2D884 1995 552'.4'09798--dc20 95-34356
CIP
For sale by U.S. Geological Survey, Information Services, Box
25286, MS 306, Federal Center, Denver, CO 80225
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CONTENTS
Page Abstract - - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - . - - - - D l Introduction
......................................................................
1
Acknowledgments ----.----------------------------------- 4
Summary of the major metamorphic episodes that
affected southeastern Alaska
...................................... 7 Detailed description of
metamorphic map units .................... 9
Southern Prince of Wales Island and adjacent islands ---- 9 GNS
(Oc) + LPP (DS) ........................................ 9 AMP (Oc)
+ LPP (DS) ........................................ 10 Lpp (DS) . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 GNS (DS) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 10 GNS (DS) + GNS (K)
........................................ 11
Glacier Bay and Chichagof and Baranof Islands area ----- 11 AMP
(eKR) -----------------------------------.. 11 GNS (eK17;)
----------------------------------------. 12 AMP (eKlg) . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12 GNS (eKIT)
....................................................................................................
13 Lpp (eKeJ) -.-------------------------------------- 13 AMP (eK)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 14 LPP/GNS (eTI'E;) ------------.------------------- 14 LPp GNS
(eTIJ) ----------..---------------------- 15 LPP GNS (eTIJ) + GNL
(eT) ..................................... 16 LPP GNS (eTIJ) + AML
(eT) ..................................... 17 GNS/AMp (eTK) . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 18 AMP (eTK) . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 19 LPP (eTIK) - -
-------------------------------- . 19
Detailed description of metamorphic map units-Continued Western
metamorphic belt ........................... --------------
Admiralty Island and adjacent mainland area ------ Lpp (eK)
----------.------------------------------------------ GNS (eK)
.................................................... GNS AMP (eK)
.............................................
Kupreanof, Etolin, and Revillagigedo Islands and Cleveland
Peninsula area ..........................
Lpp/GNS (mK) .............................................
LPP/GNS (mK) + GNL +I (IK) ......................... GNS (K)
...................................................... AM1 (IK)
......................................................
Mainland belt
.................................................... GNS (eTIJ)
-.------------------------------------------------ GNL +I (IK) +
GNI (eTIK) ............................... LPP (eTIK) ....
.............................................................................
1 - -.- GNI (eTIK)
................................................... GNI AM1 (eTIK)
............................................ AM1 (eTIK)
................................................... AM1 L (eTIK)
................................................
Tectonic interpretation of metamorphism in the western
metamorphic belt during Late Cretaceous and early Tertiary time
------------
References cited
...............................................................
ILLUSTRATIONS
[Plates are in pocket]
PLATE 1. Metamorphic facies map of southeastern Alaska 2.
Metamorphic-mineral locality map of southeastern Alaska
Pdge
FIGURE 1. Map showing area of this report and other reports in
the series of metamorphic studies of
Alaska------------------------------ D2 2. Map showing regional
geographic areas in southeastern Alaska that are discussed in text
........................................ 3 3. Diagram showing
schematic representation of metamorphic-facies groups and series in
pressure-temperature
space and their letter symbols
.........................................................................................................................
4 4. Map showing general sources of metamorphic data for the
metamorphic facies map of southeastern Alaska ----------------
6
TABLES
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REGIONALLY METAMORPHOSED ROCKS OF ALASKA --
METAMORPHIC FACIES MAP OF SOUTHEASTERN ALASKA - DISTRIBUTION,
FACIES, AND AGES OF REGIONALLY
METAMORPHOSED ROCKS
By CYNTHIA DUSEL-BACON, DAVID A. BREW, and SUSAN L. DOUGLASS
ABSTRACT
Nearly all of the bedrock of southeastern Alaska has been
metamorphosed to some degree. Much of it has been metamor- phosed
under medium- to high-grade conditions during episodes that were
associated with widespread plutonism.
The two oldest known metamorphic episodes in southeastern Alaska
occurred during an early Paleozoic and a middle Paleo- zoic orogeny
and affected probable arc-type volcanic, sedimenta- ry, and
plutonic rocks in the area of southern Prince of Wales Island.
During Late Cambrian to Early Ordovician time, rocks were
penetratively deformed, flattened, and recrystallized under
greenschist- to amphibolite-facies conditions. The subsequent
metamorphic episode, which was Silurian to earliest Devonian in
age, occurred under prehnite-pumpellyite-facies conditions and was
not accompanied by penetrative deformation.
The predominant period of metamorphism and plutonism oc- curred
during the interval of Early Cretaceous to early Tertiary time. The
oldest documented metamorphic episode during this interval took
place in northern southeastern Alaska and appar- ently was
associated with the intrusion of elongate bodies of highly
foliated, 120- to 110-Ma tonalite and diorite. Low-grade
metamorphism of mid-Cretaceous age produced a weakly to moderately
developed metamorphic fabric in rocks extending from Kupreanof
Island in the north to the peninsula north and west of
Revillagigedo Island (Cleveland Peninsula) and perhaps to
Revillagigedo Island in the south. This episiode predated the
intrusion of mafic-ultramafic bodies that have yielded K-Ar ages of
110-100 Ma. Low-grade metamorphism of melange and flysch north of
Cross Sound and on Chichagof and Baranof Islands, southwest of the
Peril Strait fault, also took place sometime during the Early
Cretaceous to early Tertiary interval. Meta- morphism of the
melange occurred in a subduction environment and may have begun a s
early a s latest Jurassic time.
Metamorphism during the next episode or phase was associ- ated
with the intrusion of garnet- and epidote-bearing plutons of early
Late Cretaceous age (approximately 90 Ma). Experi- mental data on
the composition of magmatic garnet and the pressure required to
crystallize magmatic epidote have been used to infer a minimum
13-15 kb initial depth and a 6-10 kb final depth for
crystallization of the magma body. Kyanite, in- dicative of
intermediate-pressure conditions, occurs in exten-
Manuscript approved for publication March 26, 1987.
sively developed aureoles around some of the plutons within
amphibolite-facies rocks in the central and southern part of the
90-Ma metamorphic belt. Other 90-Ma plutons that intrude low- grade
rocks have narrow low-pressure aureoles. Relict andalu- site that
has been replaced by kyanite occurs in aureoles within
amphibolite-facies rocks near the northern end of the 90-Ma belt,
near northernmost Wrangell Island. The early formation of
andalusite, indicative of low-pressure conditions, is hard to rec-
oncile with the high to intermediate pressures inferred for the
associated garnet- and epidote-bearing plutons.
The final metamorphic episode or phase was mostly synkin- ematic
with, but may have slightly preceded, the lastest Creta- ceous and
early Tertiary mesozonal intrusion of a 600-km long,
northwestward-trending composite body herein referred to as the
"great tonalite sill." The northern two-thirds of the meta- morphic
belt produced in Alaska during this episode is com- posed of an
intermediate-pressure (Barrovian) sequence in which metamorphic
grade increases northeastward. Isograds marking the first
appearance of biotite, garnet, staurolite, kya- nite, and
sillimanite are generally parallel to the sill and in- verted.
Kyanite has not been reported in the southern part of the belt, and
metamorphic pressures may not have been as high as in the north.
Intermediate and felsic epizonal plutons in- truded the eastern
part of the belt in Eocene time during the final phase of this
metamorphic episode and produced low-pres- sure aureoles and zones
of migmatite.
INTRODUCTION
This report identifies and describes the major, regionally
developed metamorphic episodes that af- fected southeastern Alaska
throughout its evolu- tion. It is one of a series of four reports
that pre- sents the metamorphic history of Alaska (fig. 1).
Metamorphic rocks are assigned to metamorphic- facies units, which
are shown on a 1:1,000,000- scale colored map (pl. I), on the basis
of the occur- rence of pressure- and temperature-sensitive min-
erals and the age of metamorphism. Plutonic rocks are also
categorized on the basis of their age and the relation of their
intrusion to metamorphism. By means of detailed unit descriptions,
this report
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D2 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
summarizes the present (about mid-1987) state of knowledge of
the metamorphic grade, pressure con- ditions, age of protoliths and
metamorphism, and, in some cases, the speculated or known tectonic
origin of regional metamorphism. Metamorphic units are discussed in
the same order as that used for the map explanation. Within each
geographic area (fig. 2), units are discussed in chronologic or-
der, from oldest to youngest; units of the same metamorphic age or
age range are discussed in or- der of increasing metamorphic
grade.
The metamorphic-facies determination scheme (fig. 3, table 1) on
which the map (pl. 1) is based was developed by the Working Group
for the Car- tography of the Metamorphic Belts of the World (Zwart
and others, 1967). This scheme is based on pressure- and
temperature-sensitive metamorphic minerals that are
petrographically identifiable by most geologists. Regionally
metamorphosed rocks
are divided into three facies groups based on in- creasing
temperature: (1) laumontite and prehnite- pumpellyite facies (LPP),
shown in shades of gray and tan; ( 2 ) greenschist facies (GNS),
shown in shades of green; and (3) epidote-amphibolite and
amphibolite facies (AMP), shown in shades of red and orange. Where
possible, the greenschist-facies and the epidote-amphibolite- and
amphibolite- facies groups are divided into three facies series
based on pressure. A high-, intermediate-, or low- pressure-facies
series is indicated by an H, I, or L in place of the final letter
in the symbol used for the previously mentioned facies groups.
In this compilation, the scheme of Zwart and others. (1967) is
expanded. Specifically, combina- tions of letters and symbols are
used to indicate metamorphic conditions transitional between dif-
ferent facies groups and series. Where two facies groups or facies
series occur together but have not
FIGURE 1.-Map showing area of this report (shaded) and other
reports in the series of metamorphic studies of Alaska. A,
Dusel-Bacon and others (1989); B, Dusel-Bacon and others (1996); C,
Dusel-Bacon and others (1993).
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SOUTHEASTERN ALASKA
50 25 0 50 KILOMETERS I I I I
FIGURE 2.-Regional geographic areas in southeastern Alaska that
are discussed in text.
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D4 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
been differentiated, the designation of the more abundant facies
is given first, and the two designa- tions are separated by a
comma. Where the meta- morphic grade of a unit was transitional
between two facies groups, the lower grade designation is given
first, and the two designations are separated by a slash. Where the
pressure changes within a facies series, an arrow is used to show
the direc- tion of change. As a further expansion, a symbol for
either the metamorphic age or minimum and maximum limits of the
metamorphic age is given in parentheses following the facies
symbol. In two instances, the numerical subscript "1" is used to
differentiate between map units that have the same metamorphic
grade and age but that have different protoliths and are believed
to have differ- ent metamorphic histories. Where two metamor- phic
episodes have affected the rocks, the symbol gives the facies and
age of each metamorphic epi- sode, beginning with the oldest
episode. Protolith and metamorphic age designations are based on
the Decade of North American Geology Geologic Time Scale (Palmer,
1983). Radiometric ages cited
herein have been calculated or recalculated using the decay
constants of Steiger and Jager (1977).
Metamorphic mineral assemblages for most metamorphic-facies
units (table 2) follow the de- tailed descriptions of the
metamorphic units and are keyed to the metamorphic-mineral locality
map (pl. 2).
General sources of metamorphic data used to com- pile the
metamorphic facies map (pl. 1) are shown on figure 4. Complete
citations for published sources are given in the references.
Additional sources are re- ferred to in the detailed unit
descriptions.
ACKNOWLEDGMENTS
We wish to thank the numerous geologists from the U.S.
Geological Survey, the State of Alaska De- partment of Natural
Resources, Division of Geo- logical Surveys, and several
universities who freely communicated their thoughts and unpublished
data to this report. R.A. Loney, M.L. Crawford, G.E. Gehrels, R.D.
Koch, and R.L. Elliott were par-
1 FAClES GROUPS I FACES SERIES I TEMPERATURE - TEMPERATURE I
- High-pressure-facies series
FIGURE 3.-Schematic representation of metamorphic-facies groups
and series in pressure-temperature space and their letter symbols
used in this report (modified from Zwart and others, 1967).
Stability fields of AI,SiO, polymorphs andalusite (anda.), kyanite
(ky.), and sillimanite (sill.) shown by dashed lines.
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SOUTHEASTERNALASKA
Table 1.-Scheme for d e t e r m i n i n g metamorph ic
facies
[Modified from Zwart and others, 19671
Facies symbol
Diagnostic minerals Forbidden minerals C o m m o n a n d
assemblages and assemblages minerals and
assemblages
Remarks
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LAUMONTITE AND PREHNITE-PUMPELLYITE FACIES
LPP Laumontite +quartz, Pyrophyllite, analclme + "Chlorite",
saponite, Epidote, actinolite, and prehnite + purnpellyite. quartz,
heulandlte dolomite + quartz. "sphene" possible in
ankerite + quartz, prehnite-pumpellyite kaolinite, montmoril-
facies. lonite, albite, K-feldspar. "white mica"
GREENSCHIST FACIES
GNS
GNL and GNI
GNH
GNH (w~th
stipple)
Staurolite, andalusite. Epidote, chlorite. cordierite,
piagioclase chloritoid, albite. (An>lO), laumontite + muscovite,
calcite. quartz, prehnite + dolomite, actinolite pumpellyite
talc
Glaucophane, crossite aragonite. jadeite + quartz
Low- and intermediate-pressure greenschist facies
Hornblende. glaucophane. crossite. lawsonite. jadeite + quartz.
aragonite
High-pressure greenschist (blueschist) facies
Almandine. paragonite, stilpnomelane
Low-temperature subfacies of high-pressure greenschist
facies
Above minerals plus purnpellyite and (or1 lawsonite
Blotite and manganiferous garnet possible; stilpno- melane
mainly restricted to intermediate-pressure greenschist facies.
Subcalcic hornblende (barroisite) may occur in h~ghest
temperature part of this facies.
EPIDOTE-AMPHIBOLITE AND AMPHlBOLlTE FACIES
AMP
AML
Staurolite Orthopyroxene + Hornblende. plag~o- clinopyroxene.
clase. garnet, biotite. actlnolite + calcic muscovite. diopside.
playioclase +quartz. K-feldspar. rutile, cal- glaucophane. cite,
dolomite, scapolite
Low-pressure amphibolite facies
Andalusite + staurolite. Kyanite cordierite + orthoamphl-
bole.
Cordierite. sillimanite. cummingtonlte.
Pyralspite garnet rare In lowest possible pressure part of this
facies.
Intermediate- and high-pressure amphibol i te facies
AM1 and AMH Kyan~te + staurollte Andalusite Sillimanite mainly
re- stricted to intermediate- pressure amphibolite facles.
TWO-PYROXENE FACIES
2PX Orthopyroxene + Staurollte, orthoamphi- Hypersthene,
clinopyrox- Hornblende possible clinopyroxene bole. muscov~te.
epidote. ene, garnet, cordler~te. Kyanite may occur in high-
zois~te anorthite, K-feldspar. er pressure part of this
sillimanite, biotite, facies and periclase and scapolite, calcite.
dolo- wollastonite in low- mite. rut~le. pressure part.
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REGIONALLY METAMORPHOSED ROCKS OF ALASKA
FIGURE 4.-General sources of metamorphic data for the
metamorphic facies map of southeastern Alaska (pl. 1). Numbers
refer to sources of data listed in explanation. Ring patterns
around numbers correspond to boundary pattern used to delineate
that area.
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SOUTHEASTERN ALASKA D7
ticularly helpful in this regard. Drafting and tech- nical
assistance were provided by M.A. Klute, E.O. Doyle, and K.E.
Reading. R.A. Loney and M.L. Crawford made valuable suggestions tha
t helped improve the original version of this manuscript. The
expert and patient map and text editing by J.S. Detterman is
especially appreciated.
SUMMARY OF THE MAJOR METAMORPHIC EPISODES THAT AFFECTED
SOUTHEASTERN
ALASKA
The oldest metamorphic episode in southeastern Alaska occurred
during Late Cambrian and Early Ordovician time under greenschist-
and amphibolite- facies conditions and produced greenstone, mafic
schist, pelitic schist, phyllite, marble, and intermedi- ate
metaplutonic rocks, all of which are assigned to the Wales Group.
These rocks crop out on southern Prince of Wales Island and
adjacent islands and have been described most recently by Gehrels
and Saleeby (1987b). I n most areas, protolith features are
obscured by penetrative metamorphic recrystalli- zation and a high
degree of flattening.
A weakly to moderately developed Silurian to earliest Devonian
metamorphic episode was part of the subsequent orogenic event tha t
affected the rocks in the vicinity of southern Prince of Wales
EXPLANATION
1. George Plafker and Travis Hudson (unpublished mapping,
1978)
2. Brew (1978) 3. Plafker and Hudson (1980) 4. MacKevett and
others (1974) 5. Redman and others (1984) 6. Barker and others
(1986) 7. Brew and Ford (1985) 8. Ford and Brew (1973) 9. Brew and
Ford (1977)
10. Ford and Brew (1977a) 11. Brew and Grybeck (1984) 12.
Lathram and others (1965) 13. Johnson and Karl (1985) 14. Loney and
others (1975) 15. D.A. Brew and D.J. Gr~beck (unpublished mapping,
1969) 16. Brew and others (1984); S.L. Douglass (unpublished
meta-
morphic facies map, 1985) 17. R.L. Elliott and R.D. Koch
(unpublished mapping, 1979) 18. Eberlein and others (1983) 19.
Gehrels and Berg (1984); G.E. Gehrels (written commun.,
1985) 20. Berg and others (1978; 1988) 21. M.L. Crawford
(written commun.. 1984)
Island (Klakas orogeny of Gehrels and others (1983) and Gehrels
and Saleeby (198713)). Rocks metamorphosed for the first time
during this epi- sode include basaltic to rhyolitic metavolcanic
rocks, metasedimentary rocks, metachert, and met- alimestone of
late Early Ordovician to Early Siluri- an protolith age and quartz
dioritic metaplutonic rocks of Middle Ordovician to Early Silurian
age (Eberlein and others, 1983; Gehrels and Berg, 1984).
Metamorphic grade is lowest (prehnite-pum- pellyite facies) on
southern Prince of Wales Island where the rocks are not
penetratively deformed, and relict sedimentary and volcanic
textures are widespread. Metamorphic grade increases south-
westward to lower greenschist facies and eastward to greenschist
facies and , locally, epidote- amphibolite facies. The Wales Group
is presumed to have undergone weak retrograde metamorphism during
this episode.
The dominant period of orogenic activity in southeastern Alaska
took place from Early Creta- ceous to early Tertiary time and
consisted of mul- tiple episodes of plutonism and dynamothermal
metamorphism. In the northern half of southeast- e rn Alaska,
metamorphism of uni ts t h a t have metamorphic-age designation
"eK" apparently was associated with the intrusion of elongate
bodies of highly foliated tonalite and diorite of Early Creta-
ceous age (120-110 Ma; Loney and others, 1967; Decker i n d
Plafker, 1982). ear Glacier Bay and on Chichagof Island, the Early
Cretaceous meta- morphic sequence consists of metasedimentary and
metavolcanic rocks of Silurian to Devonian proto- lith age that
were metamorphosed under amphibo- lite- or
hornblende-hornfels-facies conditions. In these areas, structural t
rends in metamorphic rocks parallel those of the Early Cretaceous
plu- tons. On Admiralty Island and the adjacent main- land, the
Early Cretaceous metamorphic sequence comprises metasedimentary,
metavolcanic, and metaplutonic rocks of Ordovician to Early Creta-
ceous protolith age. Metamorphic grade in the area of Admiralty
Island increases progressively toward the Early Cretaceous plutons;
in the highest grade part of the sequence, large areas of
dynamother- mally metamorphosed phyllite, schist, and gneiss merge
with contact aureoles of the plutons (Loney and others, 1967).
Low-grade metamorphism of mklange and flysch nor th of Cross
Sound and on Chichagof and Baranof Islands southwest of the Peril
Strait fault took place prior to the intrusion of Eocene plutons
and associated regional thermal metamorphism. Deposition of the
mklange is interpreted to have
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D8 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
occurred, in part, during Late Jurassic to Early Cretaceous time
(Brew and others, 1988), and deposition of the flysch is inferred
to have occurred sometime during the Cretaceous. Metamorphism of
the melange occurred in a subduction environment and may have begun
as early as latest Jurassic time. East of the Chatham Strait fault,
low-grade metamorphism of presumed mid-Cretaceous age produced a
weakly to moderately developed meta- morphic fabric. This episode
predated the intrusion of mafic-ultramafic bodies that have yielded
K-Ar ages of 110-100 Ma (Lanphere and Eberlein, 1966; Brew and
others, 1984).
In many areas, the low-grade fabric developed during
mid-Cretaceous time was subsequently overprinted by metamorphism
associated with the early Late Cretaceous (about 90-Ma) intrusion
of intermediate plutons that contain primary garnet and epidote.
Zen and Hammarstrom (1984b), using experimental data on the
composition of magmatic garnet and on the pressure required to
crystallize magmatic epidote, propose that the magma for these
plutons began to crystallize at a minimum pressure of 13-15 kb
(about 40-50 km in depth) and finally crystallized a t about 6-10
kb (about 20-30 km in depth).
The degree and regional extent of recrystalliza- tion associated
with this plutonism decreases to the north and west as does the
apparent pressure during metamorphism. In the central and southern
part of the belt (on Wrangell Island, Cleveland Peninsula, and
northern Revillagigedo Island), kyanite, which i s indicative of
intermediate- pressure metamorphic conditions, is common in
staurolite+garnet+sillimanite-bearing pelitic schist adjacent to
the larger 90-Ma plutons. In that area, metamorphic grade increases
toward the plutons and sillimanite and kyanite isograds are
concentric to large plutons. Geobarometric data from two samples of
garnet-kyanite schist from northern Re- villagigedo Island indicate
a final equilibration pressure for mineral rims of about 7 to 9 kb,
which falls in the range of pressures proposed for the
garnet-epidote-bearing plutons.
Farther north near northernmost Wrangell Is- land, pelitic
schist from the aureoles of small 90- Ma plutons contains relict
andalusite that has been replaced by static (radial) kyanite or
aggre- gates of intergrown kyanite and staurolite. The combination
of the high- to intermediate-pressure magmatic and crystallization
history inferred for the plutons and the occurrence of relict
andalusite, which is indicative of low-pressure conditions (less
than 3.8 kb; Holdaway, 1971), in their aureoles is
indeed problematic. However, within lower grade rocks to the
north and west of the amphibolite- facies rocks on Wrangell Island
and to the south and west of the amphibolite-facies rocks on
Revillagigedo Island, 90-Ma plutons that have nar- row,
low-pressure aureoles also occur (not shown or described in this
report).
The final metamorphic episode or phase of a pro- longed
metamorphic cycle, may have slightly pre- ceded but was mostly
synkinematic with the lastest Cretaceous and early Tertiary
mesozonal intrusion of a 600-km-long composite body here re- ferred
to as the "great tonalite sill." Intrusion of the sill has been
dated by U-Pb zircon methods at about 69 Ma near Juneau in the
north, at about 62 Ma near Petersburg in the central part of the
sill's extent (Gehrels and others, 1984), and a t about 58-55 Ma
east of Revillagigedo Island in the south (Berg and others, 1988).
The northern two-thirds of the metamorphic belt shown in Alaska is
composed of an intermediate-pressure (Barrovian) sequence whose
metamorphic grade increases northeast- ward. This sequence
comprises metasedimentary and metavolcanic rocks, metalimestone,
metachert, schist, amphibolite, gneiss, and migrnatite that crop
out along the mainland of southeastern Alaska from Skagway to the
area east of Wrangell Island. Min- eral isograds marking the first
appearance of biotite, garnet, staurolite, kyanite, and sillimanite
trend north-northwest, generally parallel with the elongate
tonalite sill. Isogradic surfaces dip moderately to steeply
northeast (Ford and Brew, 1973; 1977a; Brew and Ford, 1977) and,
hence, are inverted. The southern one-third of the belt in Alaska
is made up of high-grade migmatite, massive to foliated or gneissic
batholiths, and smaller plutons that enclose metamorphic screens
and roof pendants of paragneiss. Kyanite has not been reported in
the southern part of the belt, and metamorphic pres- sures may not
have been as high as in the north.
Evidence for the association of metamorphism and plutonism
during this episode consists of the in- crease in metamorphic grade
toward the sill, the general parallelism between the sill and
isograds that define the Barrovian sequence, and the parallel- ism
of foliation, contacts, and locally developed linea- tion of the
sill with structural elements in the adja- cent metamorphic rocks.
During the final phase of this episode, intermediate and felsic
epizonal plu- tons intruded the eastern part of the belt during
Eocene time and produced low-pressure aureoles and zones of
migmatite.
The tectonic environment of the widespread plutonometamorphic
episode(s) that occurred along
-
SOUTHEASTERN ALASKA D9
the western edge of the Coast Mountains in early Late Cretaceous
and early Tertiary time was domi- nated by crustal thickening
caused by the accretion of an outboard terrane(s) to the west.
Petrologic and isotopic data from metamorphic and plutonic rocks
near Prince Rupert, British Columbia, which are -considered to be
correlative with Alaskan rocks in the area of Revillagigedo Island
and the main- land to the east, have been interpreted to indicate
that crustal thickening resulted from west-directed tectonic
stacking of crustal slabs along east-dipping thrusts, in places
possibly lubricated by the intru- sion of the epidote-bearing
plutons discussed above (Crawford and others, 1987). Rapid (1
mmlyr) vertical uplift in the eastern half of the plutono-
metamorphic belt is proposed to have followed be- tween about 60
and 48 Ma, when the tonalite sill was intruded a t deep levels
during the early stages of uplift, and the felsic and intermediate
plutons were intruded at high levels during the late stages of
uplift (Hollister, 1982; revised in Crawford and others, 1987).
According to Crawford and others (1987), the intrusion of the
tonalite sill facilitated uplift by weakening the crust and serving
as a melt-lubricated shear zone.
DETAILED DESCRIPTION OF METAMORPHIC MAP UNITS
SOUTHERN PRINCE OF WALES ISLAND AND ADJACENT ISLANDS
Dynamothermally metamorphosed greenschist- facies greenschist,
greenstone, pelitic schist and phyllite, quartz-sericite schist,
and marble crop out on southern Prince of Wales Island (Gehrels and
Saleeby, 198713) and in an area (too small to show on pl. 1) on the
southernmost tip of Gravina Island and the small islands adjacent
to it (Gehrels and others, 1987). On Prince of Wales Island, this
unit also includes small areas of amphibolite-facies schist and
metaplutonic rocks in the northeastern (Herreid and others, 1978)
and southwestern (Gehrels and Saleeby, 1987b) parts of the unit.
Protoliths are mafic to intermediate volcanic rocks and
volcaniclastic strata, locally interlayered lime- stone, and minor
dioritic bodies, all of which are interpreted to have formed in a
volcanic arc environment (Gehrels and Saleeby, 1987b, and ref-
erences cited therein). The metavolcanic and meta- sedimentary
rocks have been described as the Wales Series (Brooks, 1902), the
Wales Group
(Buddington and Chapin, 1929; Herreid and others, 1978; Eberlein
and others, 1983), and most recent- ly as the informally designated
"Wales metamor- phic suite" (Gehrels and Saleeby, 1987b). Prelimi-
nary U-Pb data on zircon indicate Middle and (or) Late Cambrian
protolith ages for interlayered metaplutonic bodies and, therefore,
Late Protero- zoic and (or) Cambrian protolith ages for the meta-
sedimentary and metavolcanic rocks (Gehrels and Saleeby,
1987b).
The most abundant and widely distributed rock type is
greenschist (chlorite+albite+epidote f quartz* actinolite); more
siliceous and argillaceous variants contain sericite rather than
chlorite. Metamorphosed silicic volcanic rocks (quartz ke-
ratophyre) are commonly associated with the greenschist and have a
distinctive blastoporphy- ritic texture defined by quartz eyes and
pheno- crysts or glomeroporphyritic clots of albite in a
fine-grained matrix of quartz and albite. On the basis of
petrographic and chemical evidence, Herreid and others (1978)
proposed that although much of the albite in the metamorphosed
kerato- phyre and spilite is of igneous origin, some of the albite
present in those rocks, like that present in metasedimentary
phyllites and greenschists, is of metamorphic origin.
In most areas protolith features are obscured by penetrative
metamorphic recrystallization and a high degree of flattening.
Locally preserved proto- lith features include relict pillows and
pyroclastic fragments in basaltic and andesitic metavolcanic rocks
and rhythmic and graded bedding in meta- sedimentary rocks (Gehrels
and Saleeby, 1987b). Schistosity in these and the other rocks of
this unit generally is parallel to compositional layering (Herreid
and others, 1978; Eberlein and others, 1983). Shallow-plunging
upright folds, which have kilometer-scale wavelengths, and
asymmetric out- crop-scale folds, which probably formed as
parasitic structures on limbs of the regional folds, do not have an
axial planar foliation and are interpreted to have formed during
the waning stages or after the main phase of deformation and
metamorphism (Gehrels and Saleeby, 1987b).
Metamorphism of the Wales Group took place during the Late
Cambrian and Early Ordovician Wales orogeny of Gehrels and Saleeby
(1987a, b) as indicated by the following data: (1) metaplutonic
rocks of Middle and (or) Late Cambrian age are metamorphosed and
deformed (Gehrels and Saleeby, 1987b); (2) uppermost Lower and
Middle Ordovician marine clastic strata of the Descon For- mation
that occur near and probably overlie the
-
D l 0 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
Wales Group are only weakly metamorphosed and lack the
penetrative metamorphic fabric character- istic of the Wales Group
(Eberlein and others, 1983; Gehrels and Saleeby, 1987b); and (3)
K-Ar data indicate that the Wales Group was involved in a regional
thermal event that cooled to argon- blocking temperatures about 483
Ma (Turner and others, 1977). This unit does not appear to have
been retrograded by subsequent low-grade meta- morphism, but
geologic relations indicate that it was probably affected by the
same low-grade meta- morphic episode during Silurian and earliest
Devo- nian time that is recorded in the adjacent Ordovi- cian and
Silurian rocks of unit LPP (DS).
AMP (OC) + LPP (DS)
Amphibolite-facies schist and gneiss that crop out on southern
Dall Island are interpreted by G.E. Gehrels (oral commun., 1987) to
compose higher grade equivalents of the greenschist-facies rocks of
the Wales Group to the east (unit GNS (OC) + LPP (DS)) and to have
been metamorphosed initially during the Late Cambrian and Early
Ordovician Wales orogeny described above. Protoliths are ma- rine
clastic strata, basaltic and felsic volcanic rocks, and subordinate
limestone (Gehrels and Berg, 1984) of probable Late Proterozoic and
(or) Cambrian age (G.E. Gehrels, oral commun., 1987).
Characteristic metamorphic mineral assemblages include sillimanite,
garnet, muscovite, biotite, and hornblende (Eberlein and others,
1983; G.E. Gehrels, oral commun., 1985). This unit does not appear
to have been retrograded by subsequent low-grade metamorphism, but
geologic relations in- dicate that it was probably affected by the
same low-grade metamorphic episode during Silurian and earliest
Devonian time that is recorded in the adjacent Ordovician and
Silurian rocks of unit LPP (DS).
LPP (DS)
This unit comprises (1) uppermost Lower Ordo- vician to Lower
Silurian weakly metamorphosed subgreenschist- (probably
prehnite-pumpellyite-) facies metagraywacke, metamudstone,
argillite, metavolcanic rocks of basaltic, andesitic, and felsic
composition, and minor metachert of the Descon Formation and
age-equivalent rocks; (2) Silurian metalimestone on northern Dall
Island (Eberlein and others, 1983; Gehrels and Berg, 1984); and
(3)
Middle Ordovician to Early Silurian plutonic rocks (shown as a
weakly metamorphosed pluton, pl. I), which include diorite, quartz
diorite, granodiorite, quartz monzonite, granite, and subordinate
gabbro, pyroxenite, trondhjemite, quartz syenite, and mafic dikes
(Eberlein and others, 1983; Gehrels and Saleeby, 1987b). The
plutonic rocks show no visible effects of the low-grade metamorphic
episode, but they are presumed to have been metamorphosed because
the proposed age of low-grade metamor- phism postdates the
protolith ages of these rocks. Low-grade metamorphism of
metasedimentary and metavolcanic rocks is indicated in the southern
area by fine-grained, felty aggregates of chlorite, albite, and
epidote; this metamorphism decreases northward (G.E. Gehrels, oral
commun., 1985). This unit is not penetratively deformed, and relict
sedimentary and volcanic textures are widespread (Eberlein and
others, 1983).
A Silurian and earliest Devonian metamorphic age is proposed on
the basis of (1) strata of Early Silurian age are metamorphosed but
overlying strata of middle Early Devonian age and younger are
unmetamorphosed (Gehrels and others, 1983) and (2) geologic
relations in greenschist-facies units GNS (DS) and GNS (DS) + GNS
(K) (dis- cussed below) that are presumed to have been
metamorphosed during the same episode that af- fected unit LPP
(DS). This metamorphic episode is considered to have been part of a
metamorphic, deformational, and mountain-building event re- ferred
to as the Klakas orogeny by Gehrels and others (1983) and Gehrels
and Saleeby (198713).
GNS (DS)
This unit comprises lower greenschist-facies greenschist,
semischist, phyllite, and slate, as well as metalimestone on Long
and Dall Islands (Eberlein and others, 1983; Gehrels and Berg,
1984). Protoliths for these rocks include Silurian limestone, Lower
Ordovician through Lower Siluri- an sedimentary and volcanic rocks
of the Descon Formation, and Middle Ordovician to Silurian in-
trusive rocks (Eberlein and others, 1983; Gehrels and Berg, 1984).
A Silurian and earliest Devonian metamorphic age is proposed for
this unit because rocks of Silurian age are involved in the
metamor- phism and are cut by an undeformed latest Siluri- an to
earliest Devonian (408k10-Ma) pyroxenite in the Dixon Entrance
quadrangle (Eberlein and 0th-
, ers, 1983; G.E. Gehrels, unpub. mapping, 1984). I Metamorphism
apparently decreases in grade to
-
SOUTHEASTERN ALASKA D l 1
the north into unit LPP (DS) (Eberlein and others, 1983).
GNS (DS) + GNS (K)
Polymetamorphosed greenschist-facies and, lo- cally,
epidote-amphibolite-facies metavolcanic and metasedimentary rocks,
metadiorite (shown as strongly metamorphosed plutons, pl. 1) and
meta- trondhjemite (shown as weakly metamorphosed plutons, pl. 1)
(Berg, 1972, 1973; Berg and others, 1978; Gehrels and others, 1983,
1987) crop out on Gravina, Annette, and Duke Islands and near Cape
Fox to the southeast. Volcanic and sedimentary protoliths are
Ordovician to Early Silurian in age; crosscutting intrusive rocks
yield Ordovician and Early Silurian (diorite) and Late Silurian
(trond- hjemite) U-Pb igneous crystallization ages on zir- con
(Gehrels and others, 1987). Greenschist-facies rocks include
greenstone, fine-grained greenschists and quartz-rich schists
(perhaps originally inter- bedded quartz keratophyres and
spilites), impure quartzites, phyllite, marble, massive hornblende-
and quartz-metadiorite, metadioritic migmatite and agmatite, and
metabasites that probably rep- resent dikes and sills associated
with the dioritic intrusive rocks (Berg, 1972, 1973; Berg and
others, 1978; Gehrels and others, 1987).
Cataclastic textures are common throughout this unit, and,
although the rocks are locally schistose, they generally are only
weakly foliated (Berg, 1972, 1973). Metadiorite and metavolcanic
and metasedimentary rocks generally have a north-
northwest-striking foliation (Gehrels and others, 1983).
Metamorphic mineral assemblages contain, depending on their
original rock types, combina- tions of chlorite,
epidote-clinozoisite, albite, color- less to moderately pleochroic
green amphibole, sericite, quartz, calcite, dolomite, and traces of
brown biotite (Berg, 1972, 1973). Minor retrogres- sive
metamorphism is apparent in the higher grade rocks of this unit
(Berg, 1972, 1973).
The first and most intense metamorphism of these rocks predated
the deposition of the overly- ing marine strata of middle Early
Devonian age (Gehrels and others, 1983). On Annette Island, Late
Silurian trondhjemite dikes crosscut the folia- tion of
metadioritic rocks tha t were metamor- phosed during the first
episode, but the dikes have the same late-metamorphic deformational
features as their wallrocks, indicating that the trondhjemite dikes
and plutons were intruded before the final deformation that
occurred during the latter stages
of the episode (Gehrels and others, 1983, 1987). The early
episode of greenschist-facies and, locally,
epidote-amphibolite-facies metamorphism of the lower Paleozoic
rocks of this unit is therefore con- sidered to have occurred
during the Silurian and earliest Devonian metamorphic,
deformational, and mountain-building episode referred to a s the
Klakas orogeny by Gehrels and others (1983, 1987).
Near Cape Fox, the metamorphosed Late Siluri- an trondhjemite
pluton has been more completely recrystallized than have been the
other plutons of that age and composition within this unit: almost
all primary biotite has been replaced by chlorite, metamorphic.
epidote is abundant, and parts of the pluton, particularly the
northern part, have been reduced to quartzofeldspathic semischist
and schist that are difficult to distinguish from the enclosing
schistose country rocks (Berg and others, 1978, 1988). Much of the
metamorphic recrystallization of this pluton may have occurred
during the second metamorphic episode.
A Cretaceous age for the second metamorphic episode is indicated
by the presence of lower green- schist-facies metamorphic mineral
assemblages and fabrics in overlying Devonian, Triassic, Juras-
sic, and Cretaceous rocks (adjacent unit GNS (K)). This lower
greenschist-facies metamorphism pro- duced retrograde metamorphic
effects in the higher grade parts of this unit (Berg, 1972, 1973).
A K-Ar age determination of about 77 Ma on biotite from a
metamorphosed diorite on the Metlakatla Penin- sula of Annette
Island (Berg, 1973) gives a local minimum metamorphic age for this
episode.
GLACIER BAY AND CHICHAGOF AND BARANOF ISLANDS AREA
AMP (eKR)
Amphibolite- or hornblende-hornfels-facies am- phibolite,
gneiss, hornblende schist, and biotite schist, locally intercalated
with thin layers of mar- ble and calc-silicate granofels (Loney and
others, 1975; Johnson and Karl, 1985), crop out in the west-central
part of the Sitka quadrangle. Proto- liths are probably mafic
volcanic rocks and marine sedimentary rocks (Johnson and Karl,
1985). These protoliths predate the Middle and Late Jurassic (168-
to 155-Ma) K-Ar ages on biotite and horn- blende from crosscutting
diorite intrusions (Loney and others, 1967) and are thought to be
Paleozoic or Mesozoic in age. The most abundant rock type is
-
D l 2 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
a quartz-andesine-biotite-hornblende schist that commonly
contains almandine garnet; variations in the amount of quartz and
feldspar versus mafic minerals are common and give the rocks a
striking banded appearance. Calc-silicate granofels contains the
assemblages bytownite-diopside-clinozoisite- calcite-pyrite-sphene
and quartz-calcite-diopside- grossularite.
The age and origin of this metamorphic episode are unknown.
Loney considers that the metamor- phism of this unit is unrelated
to the intrusion of the Jurassic plutons because the direction of
an overall increase in metamorphic grade (generally northwest to
southeast in the vicinity of Salisbury Sound) bears no relation to
the distance from the plutons (R.A. Loney, oral commun., 1985).
How- ever, Johnson and Karl (1985) report that this unit grades
into the dioritic rocks of Jurassic and Ju- rassic or Cretaceous
age and that the metamorphic rocks become more migmatitic close to
the plutons. The relations described by Johnson and Karl imply a
genetic connection between plutonism and meta- morphism. Following
the interpretation of Johnson and Karl, these metamorphic rocks may
be part of the basement of the Jurassic and Early Cretaceous arc
that is now represented by the more volumi- nous late Early
Cretaceous granitic rocks of Chichagof Island, as suggested by D.A.
Brew and S.M. Karl (oral communs., 1985). Because of the
uncertainty in the metamorphic history of this unit, we have shown
the age of metamorphism to be sometime during Paleozoic to Early
Cretaceous time to allow for the possibilities that metamor- phism
occurred long before plutonism (assumming the oldest possible
protolith age for the rocks) or that metamorphism accompanied
plutonism.
GNS (eKIX)
Greenschist-facies chlorite schist, mica schist, phyllite,
slate, metalimestone, and subordinate im- pure quartzite crop out
in the central part of the Skagway quadrangle. Protoliths include
mafic vol- canic rocks, tuff, and quartzofeldspathic, pelitic,
calcareous, and carbonaceous sedimentary rocks that have been
correlated with rocks of Silurian to Permian age (MacKevett and
others, 1974; Redman and others, 1985; Gilbert and others, 1987).
Rocks of this unit are structurally complex; they have been
multiply deformed and locally are complexly folded (MacKevett and
others, 1974).
Schist and phyllite are fine grained and are com- posed
primarily of chlorite, quartz, muscovite, cal-
cite, and sodic plagioclase. Some chloritic schist and phyllite
also contain biotite and epidote and trace amounts of sphene or
garnet, particularly near plu- tons. Higher grade hornblende- or
staurolite-bearing assemblages also occur near intrusive bodies
(MacKevett and others, 1974). Large areas of ther- mally
metamorphosed rocks are shown on the meta- morphic facies map
(contact aureole symbol, pl. l). Temperature and pressure(?)
conditions of regional dynamothermal metamorphism probably
increased to the northeast because along the northeast margin of
this unit schist and phyllite is in gradational con- tact with
gneiss and amphibolite of the amphibolite- facies unit described
immediately below.
The age of the metamorphism is not well known. It is bracketed
between the inferred Permian age of the youngest protolith proposed
by MacKevett and others (1974) to be present in the Skagway area
and the Early Cretaceous (about 120- to 110-Ma) age of crosscutting
intermediate plutons that postdate dy- namothermal metamorphism
(MacKevett and others, 1974). Geologic evidence from the apparent
continua- tion of the metamorphic sequence about 105 km to the
northwest in Canada suggests that the regional metamorphism and
deformation occurred during lat- est Triassic to Early Cretaceous
time and may have been associated with latest Jurassic to earliest
Cre- taceous (150- to 130-Ma) plutonism. According to R.B. Campbell
and C.J. Dodds (written communs., 1986),
The younger of the 130- to 150-Ma plutons seem clearly to post-
date the metamorphism and deformation in the northeast, where they
produce distinct contact metamorphic aureoles. The older plutons of
this group to the southwest may have been in- truded during the
metamorphism and deformation; they are commonly elongate parallel
with the regional structural grain, but they clearly have local
crosscutting contacts and probably in part postdate those events.
Upper Triassic strata probably rest unconformably on Paleozoic
rocks but, nevertheless, appear to be equally deformed and
metamorphosed; thus, the widespread deformation and metamorphism
seem to be post-Late Triassic and pre-earliest Cretaceous.
Assuming continuation of the metamorphic unit and the continuity
of the general geologic relations, we have assigned Late Triassic
and Early Cretaceous metamorphic-age brackets to this unit.
AMP (eKIX)
Quartz-biotite gneiss, amphibolite, hornblende schist, and minor
phyllite and marble (MacKevett and others, 1974; Redman and others,
1985; Gil- bert and others, 1987) crop out in the central part
-
SOUTHEASTERN ALASKA D l 3
of the Skagway quadrangle. Protoliths are pre- sumed to range in
age from Silurian to Permian and include granitic rocks, from which
the gneiss probably was derived, and mafic volcanic rocks and
dikes, sedimentary rocks, and limestone, from which the other rocks
were derived (MacKevett and others, 1974). Rocks of this unit are
strongly foliated; gneissic rocks a re locally folded or
cataclastically deformed (MacKevett and others, 1974).
Probable orthogneiss contains abundant quartz
+oligoclase+biotite+muscovitefpotassium-feldspar. Less abundant
minerals include epidote, calcite, and chlorite (largely a s
alteration products), opaque minerals, and rare garnet and
staurolite (MacKevett and others, 1974). Arnphibolite is com- posed
primarily of hornblende, andesine, minor to abundant chlorite and
biotite, and minor quartz, opaque minerals, calcite, epidote, and
sphene. Hornblende-biotite-quartz schist locally contains small
amounts of chlorite and garnet (MacKevett and others, 1974).
Mineral assemblages indicate conditions of the lower amphibolite
facies.
The age and origin of metamorphism are the same as those for
unit GNS (eKI3) discussed above.
GNS (eKIli),
This unit comprises weakly metamorphosed metabasalt and, on the
west side of the Chilkat Peninsula directly south of Haines,
associated car- bonaceous argillite, metasiltstone, volcaniclastic
metasandstone, and metalimestone (MacKevett and others, 1974;
Plafker and Hudson, 1980; Redman and others, 1984; Davis and
Plafker, 1985).
Metabasalt between Klukwan and Haines is characterized by
near-vertical foliation that strikes northwest, approximately
parallel to the fault that occurs along the west margin of the unit
(MacKevett and others, 1974; Redman and others, 1984). Metabasalt
of the Chilkat Peninsula south of Haines is not penetratively
deformed, and pri- mary textures and structures are typically well
preserved (Plafker and Hudson, 1980; Davis and Plafker, 1985).
Metabasalt is recrystallized to lower greenschist-facies
assemblages that include primarily chlorite, actinolite, epidote,
clinozoisite, albite, white mica, and sphene. Geochemical data from
metabasalt on the Chilkat Peninsula and the Carnian(?) age of
associated weakly metamor- phosed limestone have been interpreted
to indicate
that these rocks were originally part of, or were co- extensive
with, the Wrangellia terrane of Jones and others (1977) (Plafker
and Hudson, 1980; Davis and Plafker, 1985).
Metamorphism is known to have occurred some- time between the
Late Triassic (Carnian?) age of the protoliths and the late Early
Cretaceous age of crosscutting plutons (MacKevett and others,
1974). Low-grade metamorphism may have been caused by heating as a
result of the intrusion of Creta- ceous dioritic or granodioritic
rocks that crop out adjacent to the metabasalt along its east
margin. The origin of the near-vertical foliation in the northern
part of this unit is unknown.
LPP (eKeJ)
This unit comprises weakly metamorphosed Si- lurian and (or)
Devonian metagraywacke, argillite, phyllite, slate, semischist,
metalimestone, meta- conglomerate, and mafic to intermediate
metavol- canic rocks; Mississippian metalimestone and shale;
Permian phyllite, slate, semischist, meta- limestone, metavolcanic
rocks, and metachert; and Lower Jurassic(?) metachert and argillite
(Lathram and others, 1959; Loney and others, 1975; Brew, 1978; C.D.
Blome, written commun., 1987). The rocks appear to be very weakly
recrystallized but are lacking minerals that are diagnostic of a
par- ticular low-grade metamorphic facies. Upper Devo- nian
andesitic and basaltic rocks on northern Chichagof Island have
experienced widespread chloritization and epidotization and
locallized albitization (Lathram and others, 1959). Metamor- phic
minerals developed in the matrix of gray- wackes on Chichagof
Island include chlorite, epi- dote, white mica, albite, and quartz
(Devonian Cedar Cove Formation) and chlorite and calcite (Si-
lurian Point Augusta Formation) (Loney and oth- ers, 1975). Permian
rocks that crop out northeast of Muir Inlet in Glacier Bay and
Lower Jurassic(?) metachert and argillite in the north-central part
of the Chilkat Range appear to have experienced the same general
degree of recrystallization as the Si- lurian and Devonian rocks
(D.A. Brew, unpub. data, 1985, 1986).
The age and origin of low-grade metamorphism are unknown.
Metamorphism is known to be brack- eted between the Early
Jurassic(?) protolith age of the youngest rocks that are clearly
metamorphosed and the late Early Cretaceous age of crosscutting
plutons. A gradual increase in metamorphic grade between the
low-grade rocks of this unit and the
-
D l 4 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
amphibolite-facies rocks (AMP (eK)) does not ap- pear to be
present on Chichagof Island; this sug- gests that, unlike the
higher grade rocks whose metamorphism is considered to have been
associ- ated with late Early Cretaceous plutonism, low- grade
metamorphism in that area may have oc- curred prior to plutonism.
In the Glacier Bay area, evidence of overprinting by the locally
adjacent AMP (eK) metamorphism is not present, however, and the
low-grade metamorphism may be a more distant expression of the same
event.
AMP (eK)
This unit constitutes a diverse assemblage of amphibolite-facies
and hornblende-hornfels-facies pelitic and semipelitic schist and
gneiss, marble, and amphibolite and minor amounts of lower grade
greenstone and greenschist. Protoliths are known to be sedimentary
and volcanic rocks of Silurian and Devonian age (Loney and others,
1975; Brew, 1978). These rocks are considered to be part of the
Alexander terrane (Brew, 1978; Decker and Plafker, 1982). The unit
is extensively intruded by elongate bodies of highly foliated
tonalite and dio- rite of late Early Cretaceous age (M.A. Lanphere,
unpub. data, 1980; Decker and Plafker, 1982) that generally trend
north-northwest; in the northern part of the unit, west of Muir
Inlet, these elongate bodies trend west-northwest and east-west
(Brew, 1978). Structural trends in metamorphic rocks par- allel
those of the Cretaceous plutons. The unit is also intruded by a
much lesser volume of un- foliated Tertiary granitoids.
North of Cross Sound, characteristic metamor- phic mineral
assemblages in metasedimentary rocks are calcic
plagioclase+hornblendefquartz fbiotitef chlorite and calcic
plagioclase+quartz +potassium-feldspar+hornblende+biotite+diopside
(Seitz, 1959). Mafic rocks generally have been recrystallized into
well-foliated and locally banded gneiss composed of
hornblende+quartz+calcic plagioclasefgarnet and poorly foliated to
unfoliated hornblende-plagioclase rock of Rossman (1963) and (Brew,
1978).
On Chichagof Island, rocks are intensely folded, and a complete
gradation in metamorphic textures between granofels or hornfels and
foliated rocks is present (Loney and others, 1975). A
characteristic metamorphic mineral assemblage in mafic rocks is
andesine+hornblende+quartzfbiotitefdiopside. Quartzofeldspathic
granofels and schist, derived from igneous rocks, contain quartz,
microcline, oligo-
clase or andesine, biotite, and minor muscovite. These rocks
also contain minor amounts of alman- dine garnet, but whether it is
a metamorphic or rel- ict igneous mineral is not known (Loney and
others, 1975). Mineral assemblages found in calcareous granofels
and marble a re calcite+diopsidef grossularite~wollastonite,
calcite+serpentine+brucite +spinel+scapolite, and
calcite+cummingtonite.
Metamorphism of this unit is considered to have taken place
during intrusion of the late Early Cre- taceous plutons. The
general parallelism between the foliate fabric of the plutons,
pluton-wallrock contacts, and structures in the wallrocks suggests
that plutonism, folding, and thermal and dynamo- thermal
metamorphism all took place as part of a continuum that occurred
under roughly the same stress conditions. K-Ar ages of about
110-120 Ma for the plutonic rocks (Loney and others, 1967; Decker
and Plafker, 1982) provide a minimum age for the late Early
Cretaceous metamorphic episode.
Transitional prehnite-pumpellyite- to green- schist-facies
greenstone, greenschist, and meta- limestone of presumed Late
Triassic age (Goon Dip Greenstone and Whitestripe Marble) and
uncon- formably(?) underlying metasedimentary and meta- volcanic
rocks of Paleozoic and (or) Mesozoic age (Loney and others, 1975;
Johnson and Karl, 1985) compose this low-grade unit. The presumed
Late Triassic age of the greenstone, greenschist, and metalimestone
is based on their correlation with lithologically similar rocks of
that age in the Wran- gellia terrane (Jones and others, 1977). The
rocks crop out adjacent to and east of the Border Ranges fault
(Plafker and others, 1976; Decker and Johnson, 1981).
Mafic protoliths of the presumed Late Triassic rocks consist of
commonly amygdaloidal basaltic flows, sills, and flow breccias.
Most massive green- stone has been recrystallized to epidote,
actinolite, chlorite, albite, prehnite, calcite, pyrite, and
sphene; relict amygdules commonly are filled with quartz and
epidote, accompanied by chlorite or prehnite. Greenschist is
composed of albite, chlo- rite, and epidote. Pumpellyite has not
been re- ported in any mafic assemblage, but this may reflect the
lack of familiarity with this mineral by those doing the early,
detailed petrography of these rocks (Rossman, 1959; Loney and
others, 1963) rather than inappropriate chemical or physical con-
ditions for the formation of this mineral (R.A.
-
SOUTHEASTERN ALASKA Dl5
Loney, oral comun., 1985). Marble is locally stylo- litic and is
composed of nearly pure calcite and, lo- cally, accessory chlorite,
sericite, graphite, quartz, albite, and pyrite (Johnson and Karl,
1985).
The low-grade rocks of Paleozoic or Mesozoic age that crop out
discontinuously along the east edge of this unit (unit MzRs of
Loney and others (1975) and Johnson and Karl (1985)) include
metachert, metasandstone, metatuff, metalimestone, and slaty
argillite, all of which are intercalated with foliated greenstone
and greenschist. In most places, the eastern part of this
assemblage is extensively in- truded by foliated granitic rocks of
Jurassic and (or) Cretaceous age and by diabase and gabbro sills.
Metamorphic mineral-assemblage data are not available for these
rocks, but general lithologic descriptions of them suggest that
they are either of prehnite-pumpellyite-facies or lower
greenschist- facies grade. These rocks are described as being more
deformed and as having a slightly higher metamorphic grade than the
overlying Goon Dip Greenstone. Although available stratigraphic
evi- dence suggests that these underlying rocks uncon- formably
underlie the Goon Dip Greenstone (Rossman, 1959; Loney and others,
1975), they are lithologically distinct from the Permian basement
rocks in stratigraphically comparable areas else- where within the
Wrangellia terrane (Johnson and Karl, 1985).
Little is known about the age and tectonic origin of the
low-grade metamorphism of this unit. Maxi- mum and minimum
metamorphic age constraints are indicated by the Triassic protolith
age of the youngest rocks and the early Tertiary(?) age (Johnson
and Karl, 1985) of the postmetamorphic granitoids that intrude
them. Most of the Jurassic plutons (B.R. Johnson, oral commun.,
1985) are fo- liated and locally highly altered and probably pre-
date metamorphism. It is not known with certainty that metamorphism
of the pre-Late Triassic rocks of this unit took place during the
same episode that affected the overlying Late Triassic rocks, but
for the time being this seems to be the most rea- sonable
assumption.
This unit consists of undifferentiated prehnite- pumpellyite-
and lower greenschist-facies bedded turbiditic metasedimentary
rocks and metavolcanic rocks (Sitka Graywacke) and a more inboard
melange unit (Kelp Bay Group). They are included in the flysch and
melange facies, respectively, of
the Chugach terrane of Plafker and others (1977). Blocks within
the melange are Triassic or Jurassic, Late Jurassic (Tithonian),
and Early Cretaceous (Valanginian) in age (Loney and others, 1975;
Plafker and others, 1976; Decker and others, 1980; Johnson and
Karl, 1985; Brew and others, 1988). Deposition of the melange
matrix is interpreted by Brew and others (1988) to have taken
place, in part, during the Late Jurassic (Tithonian) and pre-
sumably to have continued during a t least the Early Cretaceous
(age of the youngest blocks) (Decker, 1980, Johnson and Karl, 1985;
Brew and others, 1988). The depositional age of the bedded rocks is
unknown but is considered to be Creta- ceous on the -basis of
correlation with lithologically similar rocks in the Valdez Group
and Yakutat Group to the northwest (Plafker and others, 1977; Brew
and Morrell, 1979a). Eocene plutons intrude both the melange and
the bedded rocks and estab- lish a minimum age for their
protoliths. In the Fairweather Range north of Cross Sound, the unit
includes graywacke, phyllite, and minor slate, phyllite, argillite,
graywacke semischist, and meta- conglomerate (Brew, 1978; Brew and
Morrell, 1979a). On Yakobi, western Chichagof, and Baranof Islands,
the unit includes these same lith- ologies as well as greenstone
and greenschist and large areas of melange. The melange is composed
of kilometer-scale blocks of greenstone, greenschist, metatuff,
metagraywacke, meta-argillite, meta- chert, metalimestone,
phyllite, quartzite, and graywacke semischist that are separated
from one another by shear zones and faults; smaller blocks of these
same lithologies are set in a matrix of moderately metamorphosed
and penetratively sheared graywacke, argillite, and metatuff (undi-
vided Kelp Bay Group, Khaz Formation, Freeburn assemblage,
Waterfall Greenstone, Pinnacle Peak Phyllite, and related rocks)
(Loney and others, 1975; Plafker and others, 1976; Johnson and
Karl, 1985; Decker, 1980).
The part of this unit that is located between faults that occur
in the area of Tarr Inlet and Brady Glacier (Tarr Inlet suture zone
of Brew and Morrell (1979b)) comprises a similar sequence of
structurally complicated, diverse, low-grade phyl- lite, slate,
metaconglomerate, metachert, green- stone, greenschist, and marble
tha t have been proposed to correlate with (1) the melange unit de-
scribed above (Kelp Bay Group and related rocks of the Chugach
terrane) (Decker and Plafker, 1982), (2) Permian and (or) Triassic
rocks of the Wrangel- lia terrane (Brew and Morrell, 1979b), or (3)
the melange unit in the southern part of the suture
-
D l 6 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
zone and the Wrangellia rocks in the northern part (D.A. Brew,
unpub. data, 1985). Foliated, sheared, and highly altered diorite
of late Early Cretaceous age (shown as a weakly metamorphosed
pluton, pl. 1) invades the Tarr Inlet zone (Brew, 1978; Decker and
Plafker, 1982), and the fabric and al- teration in these rocks is
probably related to the low-grade metamorphism that affected the
rest of this unit.
Characteristic metamorphic mineral assembla- ges are those of
the prehnite-pumpellyite facies and, locally, those of the lower
greenschist facies. Metasedimentary rocks in the Fairweather Range
contain chlorite, white mica, and quartz, minerals tha t are only
diagnostic of low-grade metamor- phism. Metamorphosed turbidi tes
on Yakobi, Chichagof, and Baranof Islands commonly contain
metamorphic chlorite and microcrystalline epidote aggregates;
lesser amounts of quartz, white mica, albite, calcite, and prehnite
(which typically occurs in veinlets or clots) (Loney and others,
1975; John- son and Karl, 1985); and rare pumpellyite (Decker,
1980). Metamorphic assemblages and textures in the rocks of the
more inboard melange facies are more variable. Mineral assemblages
within indi- vidual blocks are those of the prehnite-pumpellyite,
lower greenschist, and blueschist(?) facies and indi- cate
low-temperature and low- to intermediate- or perhaps high-pressure
conditions (Decker, 1980).
Decker (1980) analyzed part of the melange unit on western
Chichagof Island and divided it into the following five-part
textural-grade scheme that i s based on both structural features
and increasing degree of recrystallization in argillite and thinly
layered volcanic rocks: (1) incipiently recrystal- lized, generally
massive rocks t h a t have slaty cleavage and metavolcanic rocks
that contain chlo- rite, sericite, epidote minerals, actinolite
needles, and rare prehnite and pumpellyite; (2) layered rocks that
are weakly foliated, originally massive rocks t h a t a r e still
massive, and metavolcanic rocks that are characterized by finely
disseminated chlorite, actinoli te, and epidote minerals; (3 )
subphyllitic rocks that are distinctly foliated; meta- sedimentary
rocks that contain detrital grains that have been moderately
recrystallized and have new quartz, sericite, graphite, and
carbonate; and meta- volcanic rocks that contain chlorite,
epidote-group minerals, actinolite, and, locally, blue amphibole
and phlogopite; (4) phyllitic rocks that are com- pletely
recrystallized, metasedimentary rocks that contain new micaceous
minerals, and metavolcanic rocks that contain actinolite and
micaceous miner- als; and (5) fine-grained schistose rocks in
which
metamorphic minerals a re identifiable in hand specimen for t h
e f irst time. Glaucophane and phlogopite occur in textural grades
3, 4, and 5; bar- roisite occurs only in textural grades 4 and 5;
and fuchsite and talc occur only in textural grade 5. These
textural grades were used by Decker, to- gether with
vo1canic:sedimentary rock ratios, to subdivide t h e low-grade
rocks into subduction units. Each unit is described as a piece of
oceanic crust that records i ts own subduction history which
differs from that of neighboring units. Metamor- phic textures and
mineral assemblages reflect fac- tors such as subduction rate,
depth of burial, uplift rates, and thermal history (Decker, 1980,
p. 105).
The age of low-grade metamorphism is probably Early Cretaceous
to early Tertiary, but a Late Ju- rassic age is possible. The
maximum metamorphic age is unknown, but the oldest dated part of
the melange matrix (on northern Baranof Island) con- tains fossils
of Tithonian age (Brew and others, 1988), and metamorphism may have
closely fol- lowed deposition of that part of the melange. An early
Tertiary minimum metamorphic age is indi- cated by the overprinting
of low-grade assemblages in the melange and the bedded rocks by
thermal metamorphism associated with Eocene plutons (Loney and
others, 1975; Loney and Brew, 1987). A mid-Cretaceous metamorphic
age for melange on Chichagof Island is suggested by 106- to 91-Ma
K-Ar ages on actinolite and sericite from interlay- ered
metavolcanic and metasedimentary rocks (Decker and others, 1980).
Metamorphism of this unit most likely occurred during subduction of
the Chugach terrane beneath the adjacent continental margin that,
in southeastern Alaska, was made up of the adjacent (composite)
Wrangellia-Alexander terrane.
LPP,GNS (eTIJ) + GNL (eT)
This unit consists of polymetamorphosed meta- graywacke, slate,
amphibolite, greenschist, and minor phyllite and schist whose
second meta- morphism was low- to medium-temperature, low- pressure
thermal metamorphism that was associated with Eocene plutonism.
Albite-epidote hornfels as- semblages are superimposed over the
prehnite- pumpellyite- to lower greenschist-facies assemblages
previously developed on a regional extent sometime during la tes t
Jurass ic to early Tertiary time (described above under unit
LPP,GNS (eTIJ)). These rocks are described in detail by Loney and
others (1975). Protoliths are sedimentary and volcanic rocks
-
SOUTHEASTERN ALASKA D l 7
of the Sitka Graywacke west of the northwest-south-
east-trending fault on Baranof Island and melange of the Kelp Bay
Group that contains blocks of Triassic or Jurassic, Late Jurassic,
and Early Cretaceous age east of the fault, a s described above for
unit LPP,GNS (eTIJ). The first appearance of biotite, which was
determined mainly in hand specimen, has been used to define the
contact between the regional- ly metamorphosed
prehnite-pumpellyite- to lower greenschist-facies rocks and the
polymetamorphosed rocks of this unit.
On Kruzof Island and on southern Baranof Is- land, low-grade
regional assemblages containing calcite, sphene, epidote, chlorite,
albite, and musco- vite have been replaced progressively toward the
pluton by the low-pressure assemblage epidote-
chlorite-albite-muscovite-biotite-plagioclase on into the
plagioclase-quartz-biotite-andalusite and
quartz-plagioclase-biotite-cordierite-tourmaline- garnet
assemblages of the adjacent unit LPP,GNS (eTIJ) + AML (eT).
Metamorphism during Eocene time is considered to have taken
place under low-pressure conditions that may have been transitional
into medium- pressure conditions. On southern Baranof Island,
metamorphism was accompanied by kinematic ef- fects that
intensified the preexisting foliation, pro- duced new foliation,
and developed new folds and lineations about axes subparallel to
those of the ear- lier folds. Loney and others (1975) and Loney and
Brew (1987) suggest that this area, referred to by them as
"lineated and schistose Sitka Graywacke," may have been produced
during emplacement of an inferred large igneous body that may be
present at depth between the large pluton on Baranof Island and the
smaller, elongate pluton near the southern tip of the island. This
unit along the northern mar- gin of the large pluton on Baranof
Island contains the following metamorphic mineral assemblages that
appear to indicate metamorphism was transitional between the
albite-epidote-hornfels (or greenschist) facies and the hornblende
hornfels facies: plagio- clase+quartz+biotite+epidotefchloritef
sphenekcalcite; plagioclase+actinolite+calcite+epidote;
biotite+quartz +plagioclase+almandine+albite(?)+sphene+epidote; and
plagioclase+quartz+biotite+cordierite(?)+mus- covite. Andalusite,
indicative of low-pressure condi- tions, along with cordierite and
garnet, are present in the aureole adjacent to the pluton (area too
small to show as a separate higher grade unit, pl. 1). Sedi-
mentary textures in these rocks are well preserved and relatively
unchanged to within about a hundred meters of the contact with the
pluton (Loney and others, 1975).
Rocks east of the northwest-southeast-trending fault on Baranof
Island consist of amphibolite and greenschist that are interlayered
with subordinate amounts of phyllite and biotite schist. Metamor-
phism within this part of the unit increases both to the north and
to the south as a result of proximity to two different plutons in
those directions. Typical metamorphic assemblages are
quartz-albite-epidote- actinolite (amphibolite) and
quartz-albite-epidote- chlorite (greenschist). The rocks clearly
show the remnants of an earlier cataclastic foliation.
Evidence that the second metamorphic episode is related to
Eocene plutonism consists of the relation between metamorphic
zonation and proximity to known or inferred plutons and the fact
that two of three K-Ar ages from greenschist-facies rocks of this
unit fall in the same age range (43 to 48 Ma) as was determined for
the plutons (Loney and others, 1975).
LPP,GNS (eTIJ) + AML (eT)
This unit comprises polymetamorphosed meta- graywacke and slate
hornfels, biotite schist and gneiss, and greenschist whose second
metamorphism was medium- to high-temperature, low-pressure thermal
metamorphism associated with Eocene plu- tonism.
Hornblende-hornfels-facies assemblages are superimposed over the
prehnite-pumpellyite- to lower greenschist-facies assemblages
previously de- veloped on a regional extent sometime during latest
Jurassic to early Tertiary time (described under unit LPP, GNS
(eTIJ)). These rocks are described in detail by Loney and others
(1975) and Loney and Brew (1987). Protoliths are sedimentary and
volcanic rocks of the Sitka Graywacke (west of the northwest-
southeast-trending fault on Baranof Island) and Kelp Bay Group
(east of the fault), as described for unit LPP, GNS (eTIJ).
On Kruzof Island, the contact between this unit and the adjacent
lower grade equivalents of these rocks is defined by the first
appearance in the field of readily visible garnet or cordierite in
biotite- orthoclase-plagioclase hornfels (Loney and others,
1975).
On Baranof Island, this unit south of the largest pluton is
lineated and schistose, having experi- enced the kinematic effects
presumed to have been produced during emplacement of an inferred
large subsurface pluton connecting the largest pluton shown with
the elongate one near the southern tip of the island (Loney and
others, 1975; Loney and Brew, 1987). Diagnostic metamorphic
minerals
-
D l 8 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
that mark the lower grade boundaries of this unit on southern
Baranof Island are plagioclase, alman- dine garnet, and cordierite;
andalusite occurs in a higher grade zone within this unit.
Staurolite coex- ists with andalusite and garnet in small areas in
the higher grade parts of this unit, and sillimanite occurs both
east of the elongate pluton near the tip of the island and on the
south side of the largest pluton (Loney and others, 1975). Narrow
zones of pyroxene hornfels-facies rocks (too small to show on pl.
1) occur within this unit adjacent to the Eo- cene plutons.
Metamorphic mineral assemblages in these rocks contain
quartz+plagioclase+orthoclase+ sillimanitefbiotitefenstatite (Loney
and others, 1975; Loney and Brew, 1987).
The area of this unit east of the northwest- southeast-trending
fault consists of polymetamor- phosed biotite schist and gneiss,
minor amphibolite, and quartzite. The broadness of the aureole
south of the large pluton suggests that the pluton underlies much
of the southern part of this unit. Rocks are characterized by
calcic plagioclase and almandine garnet, followed by fibrolitic
sillimanite and stauro- lite in progressively higher metamorphic
zones. With progressive metamorphism, grain size increases and a
relict cataclastic foliation becomes indistinct and is replaced by
a single schistosity (Loney and others, 1975). The presence of
staurolite, sillimanite, and al- mandine garnet in these rocks and
the absence of cordierite and andalusite suggest that pressures
within this large aureole may have been intermedi- ate between
those of the hornblende-hornfels facies and the amphibolite
facies.
Evidence for relating the second metamorphic episode to Eocene
plutonism is discussed above.
Transitional greenschist- to amphibolite-facies biotite schist
and semischist derived from gray- wacke and argillite crop out in
the Fairweather Range (Brew, 1978), and amphibolite and quartzo-
feldspathic biotite schist crop out farther to the northwest
(George Plafker, unpub. mapping, 1980). Protoliths are interpreted
to be sandstone and mudstone turbidites and volcanic rocks of
Creta- ceous age on the basis of lithologic similarity to the
Valdez Group, Sitka Graywacke, and part of the Yakutat Group
(Plafker and others, 1977; Brew and Morrell, 1979a; D.A. Brew and
George Plafker, oral communs., 1985).
Biotite schist is fined grained, variably banded, granoblastic
to strongly schistose, and composed of
quartz+biot i tef garnetfplagioclase+muscovite (Hudson and
others, 1977; Brew, 1978). Mafic metavolcanic rocks in the Skagway
quadrangle are fine to medium grained, variably segregated, and
contain the following assemblages: chlorite+white
mica+albite+epidote+quartz+sphene; plagioclase
+biotite+chlorite+quartz+actinolite+calcite; and
hornblende+epidote+plagioclase+sphene (George Plafker, written
commun., 1984).
Exact timing of metamorphism is unknown. A Cretaceous maximum
age of metamorphism is pro- posed on the basis of the age of the
protoliths. Meta- morphism is known to predate the intrusion of
cross- cutting Oligocene (28f8 Ma) gabbroic plutons north- west of
Cross Sound (Loney and Himmelberg, 1983), and probably also
predates the intrusion of Eocene plutons of intermediate
composition in the south- western Skagway quadrangle (K-Ar age on
horn- blende from one pluton is about 51 Ma and that from another
is about 45 Ma; G. Plafker, oral commun., 1989). A 67-Ma age on
hornblende from amphibolite in the Nunatak Fiord area, 55 km
northeast of Yakutat, may indicate a latest Creta- ceous
metamorphic age (Barker and others, 1985).
The origin of the metamorphic episode that resulted in the
greenschist- to amphibolite-facies metamorphism of this unit and
the adjacent amphibolite-facies unit described below is unknown. It
is doubtful that metamorphism was associated with Tertiary
plutonism because no clear cut spatial relation between higher
grade metamorphic condi- tions and proximity to Tertiary plutons
seems to be present. D.A. Brew infers that this medium-grade
metamorphic episode was the higher temperature equivalent of the
prehnite-pumpellyite- and lower greenschist-facies metamorphic
episode (unit LPP,GNS (eTIJ)) that affected the rocks north of
Cross Sound. However, a relatively high thermal gradient is
required to explain the abrupt transition from
prehnite-pumpellyite- and lower greenschist- facies rocks to
amphibolite-facies rocks observed in the western part of the
Fairweather Range. This high thermal gradient is incompatible with
the low thermal gradient implied by the subduction-related
metamorphic history proposed by Decker (1980) for the melange part
of unit LPP,GNS (eTIJ) south of Cross Sound on western Chichagof
Island (discussed above). The two areas also are not analogous in
that Decker's study concerned a simple, single gradient situation
inboard of the flysch unit, whereas the Fairweather Range contains
a complex situation in which the metamorphic grade increases
abruptly within the flysch unit from both the east and west towards
a maximum near the crest of the range.
-
SOUTHEASTERN ALASKA D l 9
AMP (eTK)
Amphibolite-facies biotite gneiss and hornblende schist and
gneiss (amphibolite) (Brew, 1978; George Plafker, unpub. mapping,
1980) crop out in the Fairweather Range. Protoliths are interpreted
to be sandstone and mudstone turbidites and volcanic rocks of
Cretaceous age on the basis of lithologic similarity to the Valdez
Group, Sitka Graywacke, and part of the Yakutat Group (Plafker and
others, 1977; Brew and Morrell, 1979a; D.A. Brew and George
Plafker, oral communs., 1985). The rocks are well foliated and
amphibolite is also locally well line- ated. Biotite gneiss is
composed primarily of biotite, quartz, plagioclase, and locally
garnet. Amphibolites contain hornblende+plagioclasefgarnetfbiotite
fchlorite (Brew, 1978; George Plafker, written com- mun.,
1984).
Constraints on the age of metamorphism and questions about i ts
origin are the same as those described for the unit immediately
above.
LPP (eTIK)
This unit comprises laumontite+quartz- and (or)
prehnite-pumpellyite-facies late Mesozoic flysch and melange of the
Yakutat block (Plafker, 1983), which consist of large blocks of
greenstone, phyl- lite, metagraywacke, argillite, and metachert and
thick sequences of slate, silty metalimestone, limy metasiltstone,
and minor greenstone (Brew, 1978). Also included is a sheared and
chloritized elongate diorite body of probable Cretaceous age (Brew,
1978; shown as a weakly metamorphosed pluton, pl. 1). The unit
crops out in the vicinity of Cape Fairweather. Northwest of the map
area, near Yakutat, correlative bedded sequences are Late
Cretaceous in age, and disrupted blocks are Late Jurassic and (or)
Early Cretaceous in age (George Plafker, written commun., 1985;
Dusel-Bacon and others, 1993).
Petrographic study of correlative sandstones to the north in the
Yakutat-St. Elias Mountains area has indicated t h a t metamorphic
minerals devel- oped in these rocks include laumontite, prehnite,
quartz, chlorite, sphene, pumpellyite, and white mica (Hudson and
others, 1977). Similar assembla- ges are inferred to exist in this
map area.
Metamorphism is known to postdate the Late Cre- taceous
protolith age of the bedded sequences in the Yakutat area (George
Plafker, oral commun., 1984) and the probable Cretaceous age of
diorite near Lituya Bay and to predate the deposition of
overly-
ing unmetamorphosed Oligocene volcanic rocks (Brew, 1978). A
latest Cretaceous and (or) early Ter- tiary metamorphic age is
provisionally assigned to these rocks on the basis of the youngest
probable protolith age (Maastrichtian) and the fact that cor-
relative low-grade rocks of the Yakutat Group that occur farther to
the north in the Yakutat-St. Elias Mountains area appear to
increase in grade into amphibolite-facies rocks (Dusel-Bacon and
others, 1993) that yield a K-Ar age on hornblende of 65 Ma (Hudson
and others, 1977).
WESTERN METAMORPHIC BELT
ADMIRALTY ISLAND AND ADJACENT MAINLAND AREA
LPP (eK)
Weakly metamorphosed argillite, slate, meta- graywacke,
metaconglomerate, intermediate to mafic metavolcanic rocks,
metachert, and minor metalimestone and phyllite compose this
low-grade unit (Buddington and Chapin, 1929; Loney, 1964; Lathram
and others, 1965; Brew and Grybeck, 1984). Protoliths are
sedimentary and volcanic rocks. On Admiralty Island, protoliths
include rocks of Ordovician age (Carter, 1977) and Permian through
Early Cretaceous age (Lathram and oth- ers, 1965). On the adjacent
mainland to the east, protoliths are correlated with rocks that
range in age from Permian through Cretaceous (Buddington and
Chapin, 1929; Brew and Grybeck, 1984). Meta- morphic minerals
developed in this unit suggest conditions of the
prehnite-pumpellyite facies and include chlorite, sericite, and
calcite in Permian metasedimentary rocks; quartz, albite, calcite,
chlo- rite, sericite, and pyrite in Triassic argillite; and
chlorite, calcite, epidote, prehnite, and white mica in Jurassic
and Lower Cretaceous metagraywacke (Loney, 1964).
Metamorphism is known to postdate the Early Cretaceous
depositional age of the youngest low- grade rocks of this unit and
to predate the early Tertiary (Paleocene to Miocene) age of
overlying unmetamorphosed sedimentary rocks. A late Early
Cretaceous age of metamorphism is proposed be- cause the low-grade
rocks of this unit grade into greenschist- and amphibolite-facies
rocks (units GNS (eK) and GNS,AMP (eK)) whose major meta- morphism
was associated with the intrusion of in- termediate plutons that
yield K-Ar mineral ages of about 120 and 110 Ma (Loney and others,
1967). Whether the area of this unit on the mainland east
-
D20 REGIONALLY METAMORPHOSED ROCKS OF ALASKA
of Admiralty Island was affected by this episode or whether
some, if not all, of the low-grade metamor- phism there was part of
the eastward-increasing metamorphic episode that occurred during
latest Cretaceous or early Tertiary time is not clear.
GNS (eK)
This unit comprises phyllite, greenschist, green- stone, slate,
marble, and metaquartz diorite; it also in- cludes poorly exposed
migmatitic rocks and feldspathic schist south of the largest pluton
on the island. The ma- jority of sedimentary and volcanic
protoliths ranges in age f?om Ordovician to Triassic (Loney, 1964;
Lathram and others, 1965; Carter, 1977); Upper Jurassic and Lower
Cretaceous sedimentary protoliths are present along the east side
of the northern peninsula of the is- land (Lathram and others,
1965). This unit is generally schistose or phyllitic. Common
metamorphic minerals are characteristic of greenschist-facies or
perhaps albite- epidote horrdels-facies conditions and include
chlorite, albite, white mica, and epidote. In the southeastern part
of this unit, phyllite, greenschist, and calcareous rocks of the
Garnbier Bay Formation are massive appearing and are phyllonitic;
these rocks contain the assemblages
quartz-muscovite-chlorite-albite, quartz-talc-calcite, and
chlorite-epidote-calcite. Augite porphyroclasts in metavolcanic
rocks in that area have overgrowths of am- phibole that consist of
colorless tremolite except for the part in contact with the augite,
which consist of a bluish (sodic) amphibole (Loney, 1964).
On the northern peninsula of the island, metaquartz diorite is
heated, well foliated, and sheared. The trend of the foliation is
parallel to that of the country rock (Barker, 1957). Barker
proposed that the pluton was in- truded &r the formation of the
schistosity in the coun- try rock and that aRer solidification the
intrusive rock was deformed and metamorphosed in a later stage of
the orogeny. However, neither the age of the igneous protolith nor
of the country rocks has been determined, and possibly either (1)
the pluton intruded unmetamorphosed country rocks and was
subsequently metamorphosed along with the country rocks or (2) it
was intruded synkmematically with regional metamor- phism. Recent
structural studies support the first inter- pretation and suggest
that the pluton was intruded as a sill-like body, was subsequently
deformed and metamor- phosed with the country rocks, and was later
sheared by movements on the Chatham Strait fault to the west (D.A.
Brew, unpub. data, 1985). Intermediate intrusive rocks of the
largest batholith on the island are poorly to well foliated, but
the trend of the foliation relative to that of the country rocks
has not been studied in detail
(Lathram and others, 1965). However, because meta- morphic grade
apparently increases toward this pluton and because the contact
aureoles merge with large areas of dynamothermally metamorphosed
phyllite, schist, and gneiss (Loney and others, 1967), we consider
that the intrusion of this pluton was associated with regional
metamorphism on Admiralty Island. A late Early Creta- ceous age for
this episode is indicated by K-A. ages of about 120 and 110 Ma on
minerals from the batholith (Loney and others, 1967). Absolute age
constraints on metamorphism are provided by the Early Cretaceous
protolith age of the youngest affected rocks and the early Tertiary
(Paleocene) age of overlying, unmetamor- phosed sedimentary
rocks.
This unit comprises undifferentiated greenschist-fa- cies (or
albite-epidote hornfels-facies) and amphibolite- facies (or
hornblende hornfels-facies) rocks, including quartz-albite-chlorite
epidoWbiotite schist, hornblende- albite-epidote hornfels,
micaceous schist, metachert, marble, slate, phyllite,
quartz-andesine-hornblende-bi- otite amphibolite,
quartz-andesine-diopside-microcline gneiss, and
andesine-garnet-pyroxene gneiss; it aIso in- cludes migmatitic
rocks north of the largest pluton on the island (Lathram and
others, 1965). Protoliths are the same as those described for the
unit above. The northern- most area of this unit is poorly exposed,
but a small granite body, whose boundary with the surrounding
gneiss is gradational across several tens of meters, crops out near
the center of the area (Lathram and others, 1965). The spatial
association between this unit and late Early Cretaceous plu-tons
and an apparent decrease in metamorphic grade away from the plutons
suggest that meta-morphism was associated with plutonism, as is
discussed above for lower grade equivalents of this late Early
Cretaceous metamorphic episode.
KUPREANOF, ETOLIN, AND REVILLAGIGEDO ISLANDS AND CLEVELAND
PENINSULA AREA
Prehnite-pumpellyite- to lower greenschist-facies greenschist,
greenstone, phyllite, slate, meta- agglomerate, and minor
argillite, semischist, metaconglomerate, metalimestone, and
m.etachert (Eberlein and others, 1983; Brew and others, 1984) crop
out from Kupreanof Island in the north to the peninsula north and
west of Revillagigedo Island
-
SOUTHEASTERN ALASKA D2 1
(Cleveland Peninsula) in the south. Protoliths in- clude clastic
sedimentary rocks, intermediate to mafic volcanic rocks, limestone,
and chert. Proto- liths